Before Jacob dies, his last official act is to bless Joseph’s sons. Joseph presents them in order, putting the oldest on Jacob’s right, and the youngest on the left, which is the proper, traditional method of delivery such blessings. The oldest should receive the blessing first and the best blessing, yet Jacob subverts expectations once again.
He switches them up. Joseph is actually pretty cross at this, even insisting that Jacob’s doing it wrong, but Jacob fires back that the younger, Ephraim, will be treater than the older, Manasseh.
So there it is. Isaac, Jacob, Joseph, and Ephraim. Four consecutive generations of children have thoroughly subverted the tradition of the oldest receiving the birthrights and blessings. While the tradition will be maintained that the oldest gets preferential treatment, there is a clear pattern of exception based on the deeds of the child rather than simple birth order.
This is further demonstrated when Judah’s tribe rises to prominence over his Reuben. Reuben’s tribe, despite him being the oldest, is obscure, receiving no special treatment. The only other special tribe is that of Levi, who become the priests of the Israelite nation post Exodus, which we’ll get to.